This invention relates to current interrupting dropout assemblies of the type adapted to span the gate between the spaced terminals of an overhead distribution cutout. More particularly, it is concerned with a dropout assembly comprising a high range current limiting fuse in combination with an expulsion-type, low range current interrupter and adapted to mate with standard size open-gate cutouts in existing distribution systems to provide full range, non-violent fault current protection without adversely affecting coordination of the system.
With increasing demands for electrical energy, utility companies have been presented the problem of efficiently distributing more and more electrical power without significantly increasing the cost of distribution. Since it is virtually impossible from the standpoint of cost to replace existing distribution networks with improved higher rated systems, the conventional approach has been to simply upgrade the current rating of the older systems. Accordingly, there is a need in the industry for various types of higher rated electrical equipment designed for compatability with existing distribution hardware.
In order to safely and efficiently transmit increased currents, the upgraded systems must be capable of withstanding higher fault currents. This requirement presents a problem with regard to the expulsion-type fusible elements conventionally used to protect electrical distribution circuits from the effects of fault currents. In this regard, expulsion fuses of this variety typically have a maximum interrupting rating of 20,000 amperes whereas fault currents of much higher values may be experienced in today's higher current rated systems. Such high magnitude fault currents can cause a violent explosion of the expulsion fuse and may even result in damage to protected devices such as transformers or the like. In rare cases, transformers have themselves blown up as a result of experiencing an extremely high fault current. Of course, such violent failures are undesirable, especially in populated residential and urban areas where significant damage to property or life could result. Electrical linemen are particularly susceptible to injury from such an occurrence since, by the nature of their job, they are sometimes in close physical proximity with such electrical equipment when a high fault current is experienced.
In order to overcome the problems associated with the use of expulsion-type fuses, many utilities have employed current limiting fuses in order to protect their distribution equipment. However, while such fuses are well suited for handling even very high fault currents, they are notorious for their failure to operate in response to low overload or fault currents. Moreover, they adversely affect desired coordination of electrical systems designed around the time-current characteristics exhibited by expulsion-type fuses. Additionally, the expensive current limiting fuses are not reusable, even after clearing only a low fault current; consequently, the cost of maintaining a distribution system protected by current limiting fuse is significantly higher than maintenance costs for a similar system protected with expulsion-type fuses.
Various attempts have been made to overcome the aforementioned problems as evidenced, for example, by the devices disclosed in the U.S. Letters Patent to Fahnoe U.S. Pat. No. 2,917,605 and Cameron et al 3,827,010. Both of these devices provide a combination dropout assembly which includes a current limiting fuse disposed in line, and electrically coupled in series, with an expulsion-type fuse such that reliable full range protection is provided by the cutout. However, the devices shown in these patents have not proved commercially successful for one very important reason. Namely, the design of the in-line, combination dropout assemblies is such that in order to provide desired current-interrupting properties, the overall length of the dropout assemblies must necessarily be longer than the gate (spacing between terminals) of most cutouts found in existing distribution systems. Thus, in order to effectively utilize the invention of Fahnoe or Cameron et al, utilities would have to replace literally millions of cutouts presently in service. Such an approach would be prohibitive not only from the standpoint of equipment cost, but also, and perhaps more significantly, in view of the monumental labor cost associated with the replacement of these cutouts.
A similar device is illustrated in the patent to Jackson et al U.S. Pat. No. 4,011,537, though in this patent the current limiting fuse and expulsion fuse are each provided with insulating skirts to overcome the flashover tendancy sometimes exhibited in devices of this type. Notwithstanding this "improvement" however, the in-line combination dropout assembly of Jackson presents the same drawbacks discussed above with respect to compatability with equipment now in service.
Another approach to overcoming the problems discussed herein above is illustrated in the U.S. Letters Patent to Mahieu et al U.S. Pat. No. 3,863,187. Mahieu employs an expulsion-type fuse in series with a current limiting fuse, but disposes the latter "outgate" such that it does not form a part of the dropout assembly. One of several advantages presented by this construction is that the size of the current limiting fuse is not dictated by the spacing between the terminals of the cutout, and moreover, the full extent of this spacing is available for accomodating the desired length of expulsion-type fuse. Thus, this arrangement permits non-violent, full range protection without adversely effecting the overall coordination of the distribution system. However, one drawback of the Mahieu device is that replacement of the current limiting fuse is difficult, particularly in adverse weather conditions. In this connection, the current limiting fuse in Mahieu is necessarily positioned on the source side of the cutout in order to provide the desired operating characteristics. Thus, linemen are usually required to work on an energized portion of the line when replacing the current limiting fuse in the Mahieu device since utilities seldom, if ever, deenergize the distribution circuit for the purposes of permitting routine maintenance work. This problem is compounded by the fact that there is no method of readily determining whether the current limiting fuse has also operated, consequently, whenever the expulsion fuse portion of the Mahieu device actuates, recommended practice is to replace both the expulsion fuse and the current limiting fuse, the latter being subsequently tested to determine whether it is suitable for continued service.